linux-kernel - Re: [RFC][PATCH 3/7] printk: introduce per-cpu alt

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Date:   Fri, 30 Sep 2016 10:05:28 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Calvin Owens <calvinowens@...com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCH 3/7] printk: introduce per-cpu alt_print seq buffer

On (09/29/16 14:26), Petr Mladek wrote:
[..]
> >  printk()
> >   local_irq_save()
> >   alt_printk_enter()
> 
> We need to make sure that exit() is called on the same CPU.
> Therefore we need to disable preemption as well.

local_irq_save() does this for us, we can't get sched tick or
re-sched IPI, and even more - we eliminate race conditions on
this CPU. only one path can touch alt_printk related stuff,
NMI works with its own buffer.

[..]
> What do you think about my approach with the printk_context per-CPU
> value from the WARN_DEFERRED() patchset? The main idea is that
> the entry()/exit() functions manipulate preempt_count-like per-CPU
> variable. The printk() function selects the safe implementation
> according to the current state.

I'll take a look.

hm, what I was thinking of... you are right, this all smells a bit
bad. I'll revisit it.

thanks!

	-ss